The present invention relates to an improved indicating device for use in a camera view finder in which an indicating member including a luminous element such as an electroluminescent device (EL) or a light emitting diode (LED) is used for automatically adjusting the luminance thereof, in association with such finder observable information as brightness of the finder viewed field, the shutter speed, the ASA sensitivity, the value of the diaphram aperture and the value of the distance setting.
In the case of information being indicated by the use of luminous elements such as the electroluminescent device (EL) and the light emitting diode (LED) as described, the luminous intensity of these elements is normally maintained at 300 cd/m.sup.2 or thereabout. Since the brightness in the visual field of finder varies from approximately 10,000 to 0.5 cd/m.sup.2 in the exposure meter follow-up range (EV17 to EV3 at ASA100), the ratio of luminance between the indicating portion and the image within the finder varies over a very wide range from several tenstimes to one-several hundredth. This results in such drawbacks that the indicating portion becomes illegible because of its excessive darkness, the image in the finder is difficult to see because of too high brightness, or a ghost image appears thus providing light which adversely effects the exposure meter operation. The present invention, in overcoming the above drawbacks provides a luminous indicating device, in which the ratio of luminance between an indicating portion within the finder and the image viewed through the finder is automatically maintained constant so as to establish optimum indication readability and image clarity. Heretofore, such a device is known which controls the luminance intensity of an electroluminescent device using light from the photographic object and in which the signal source of a fixed frequency is connected between the emitter and base of an amplifying transistor, the base thereof being biased through a photoelectric device and the primary coil of a transformer being connected as an electric load to the collector thereof so as to vary the bias voltage depending upon the resistance of the photoelectric device receiving light from the photographic object, thereby controlling the collector current, whereby the electroluminescent device (EL) connected to the transformer is controlled. In such a known device as described, the transistor is used in an unsaturated condition. In other words, the amplitude of the voltage applied to the electroluminescent device is varied, in which case the amplification ratio of the transistor greatly fluctuates depending upon the temperature, and this necessitates the provision of a special circuit for the temperature compensation. Furthermore, in the conventional device, the intensity of the light coming from a photographic object possesses an exponential functional relation to the luminance at the electro-luminescent indicating portion. On the contrary, the luminance between the light from a photographic object and the finder image is in a substantially linear proportional relationship. Accordingly, even with a slight fluctuation in the luminance of the finder image which results from a change in the intensity of light from the photographic object, the luminance at the electro-luminescent indicating portion is greatly changed, thus resulting in the illegibility of the finder image and the indicating portion.